Synthesis and characterization of molybdenum trioxide with an orthorhombic crystal structure for supercritical water gasification application

Supercritical water (SCW) catalytic gasification is promising for the production of synthetic gas from biomass. However, maintaining the activity of heterogeneous catalysts under high pressure and hydrothermal conditions is the most important. Therefore, this study investigates the activity and stability of molybdenum trioxide (MoO3) for the gasification of formaldehyde in SCW. MoO3 with an orthorhombic crystal structure was synthesized by the sol-gel method and tested in the gasification of formaldehyde in SCW. Structural and morphological changes of MoO3 with supercritical water gasification were studied by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), a scanning electron microscope (SEM), N2 adsorption-desorption, thermogravimetric analysis (TGA), temperature-programmed desorption of oxygen (O2-TPD), temperature-programmed desorption with ammonia (TPD-NH3), and temperature-programmed reaction of hydrogen (H2-TPR). The XRD results show that MoO3 has an orthorhombic crystal structure. Its structure changed to a monoclinic structure upon gasification with supercritical water. SEM images showed that the rod-shaped catalyst particles transformed into thin planar plates at high pressure and temperature conditions in the presence of carbonaceous compounds. H2-TPR and XRD results showed that MoO3 was reduced to MoO2 during SCW gasification with hydrogen and carbonaceous compounds. Moreover, the oxygen stability of the catalyst changed during SCW gasification, and active oxygen species formed on the surface of the catalyst. Compared with homogeneous gasification, in the presence of the MoO3 catalyst, the fractions of CO2 and CH4 increased to 50% and 8.4%, respectively, and the fractions of H2 and CO decreased from 37.8% and 11.8% to 32.2% and 3.9%, respectively, due to the changed decomposition mechanism. Orthorhombic molybdenum oxide can be proposed as a promising catalyst for methane production.